1. Field of the Invention
This invention generally relates to radial polymers. More particularly, the invention relates to radial polymers which have and exhibit one phase melt behavior, and to a process for the preparation of such polymers.
2. Background of the Related Art
The preparation of block copolymers is well known. In a synthetic method an initiator compound is used to start the polymerization of one monomer block. The reaction is allowed to proceed until all of the monomer is consumed resulting in a living homogeneous polymer. To this living polymer is added a second monomer block which is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization until terminated, thereby incorporating the second monomer as a distinct block into the linear polymer.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer unreactive toward monomer block or coupling agent. A polymer so terminated is commonly referred to as a diblock copolymer. Alternately, the living block copolymers can be reacted with multifunctional condensing agents commonly referred to as coupling agents. Coupling of the living ends results in linear polymers or radial polymer having at least three arms.
This synthetic approach allows the construction of materials of great practical utility. When the two blocks are sufficiently dissimilar they will not mix but will be microphase separated. This condition is to be distinguished from ordinary phase separation in that the two dissimilar materials are connected through chemical bonds. As such, the two blocks become segregated but are not allowed to migrate away from each other. This microphase separated condition may persist in both the solid and melt states.
When radial polymers possess dissimilar blocks which are rubbery and glassy, a full range of material characteristics can be achieved, from thermoplastic elastomer to impact toughened thermoplastic. If the central block is rubbery and the endblocks are glassy then the useful rubbery character of these materials arises from the constrained nature of the rubbery chains. Each end of the rubbery block is anchored in a glassy block. As a result, the mechanical energy is elastically stored in a rubber chain extension when the material is subjected to a bulk deformation. Since this microphase separated condition commonly exists in both the solid and melt states of radial block copolymers, a mechanism for elastically storing energy exists even in melts.
While the microphase separated state grants useful properties, copolymers which are microphase separated in the melt state generally have very high melt viscosities and high melt elasticities. One commercially used class of polymers having this microphase separation are KRATON G Rubbers (Trademark of Shell Oil Company) which are hydrogenated block copolymers comprising blocks of styrene and blocks of a hydrogenated diene. When melt state block copolymers are microphase separated, the high melt viscosities lead to high energy costs for block copolymer melt processing and can cause polymer degradation by requiring high melt processing temperatures. In some instances, the block copolymers cannot be melt processed in melt processing equipment unless processing aids such as oils and thermoplastic resins are incorporated. Further, the phase separated nature of the block polymer in conjunction with the high viscosities makes melt compounding of these block copolymers with other components difficult and often results in poor mixing. The high melt viscosities exclude the block copolymers from easier melt processing methods such as melt conveying and melt mixing. Furthermore, the use of melt state microphase separated copolymers in applications such as hot melt sealant and pressure sensitive adhesion (PSA) formulation are limited. A high melt elasticity also contributes to fiber fracture which further limits the use of melt state microphase separated copolymers during such applications as melt spinning of fibers.
Recent theoretical and practical work performed in this polymer field has outlined the thermodynamics controlling the microphase separated character of block copolymers. The character of the block copolymer, either as microphase separated or homogeneous, is determined by a combination of four variables: the chemical types of the constituent blocks and the resulting thermodynamic interaction between them, the molecular weights of the blocks, the relative amounts of the blocks, and the temperature. Any one variable alone is not sufficient to describe the thermodynamic state of the block copolymer and the block""s copolymer""s resulting morphology and mechanical behavior. All four variables have to be specified in order to determine the block copolymer""s thermodynamic state.
The three material variables and the temperature for determining the thermodynamic state of radial block copolymers of monoalkenyl aromatic hydrocarbons and conjugated diolefins was described in U.S. Pat. No. 5,266,648, Masse, entitled xe2x80x9cOne Phase Melt Radial Polymerxe2x80x9d which was issued on November 30, 1993. The ""648 patent discloses radial block copolymers which exhibit one phase melt behavior that provides superior viscosity properties than melt phase separated copolymers. However, the ""648 copolymers are non-hydrogenated block copolymers, and are not a substitution for the use of hydrogenated block copolymers such as KRATON G Rubbers.
Therefore, there remains a need for hydrogenated block copolymers exhibiting a one phase melt at acceptable temperatures. In particularly, it would be beneficial if the block copolymers would have low molecular weights, low melt viscosities, and low melt elasticities.
The present invention provides a radial block copolymer and a method to produce a radial block copolymer having monoalkenyl aromatic hydrocarbon blocks and hydrogenated conjugated diolefin blocks, wherein the block copolymer exhibits one phase melt behavior at temperatures below the radial block copolymer degradation temperature. The copolymer produced will have low melt viscosities and low melt elasticities allowing the copolymer to be used in melt processes such as melt conveying and melt mixing as well as in applications such as hot melt sealant and melt spinning of fibers.
In one aspect the invention provides a radial block copolymer having a plurality of arms comprising monoalkenyl aromatic hydrocarbon blocks and conjugated diolefin blocks. The monoalkenyl aromatic hydrocarbon blocks are present in an amount of from about 10 to about 40 wt % of the radial block copolymer, preferably the monoalkenyl aromatic hydrocarbon block is styrene present in an amount of from about 20 to about 30 wt %. The invention preferably includes 1,3-butadiene or isoprene as the conjugated diolefin in the composition of the arms of the copolymer. The arms of the copolymer have a numerical average molecular weight from about 5,000 to about 30,000, and preferably from about 7,000 to about 15, 000.
Another aspect of the invention provides a process for the production of the one phase melt block polymers of monoalkenyl aromatic hydrocarbon blocks and hydrogenated conjugated diolefin blocks comprising polymerizing the monovinyl aromatic, then adding the conjugated diolefin to the polymerized monovinyl aromatic to form a living block copolymer, then adding a multifunctional coupling agent to form the radial block copolymer before hydrogenating the radial block copolymer. The block copolymer produced has the one phase melt characteristics described in the preceding paragraph.
A preferred aspect of the present invention is a radial block copolymer exhibiting one phase melt behavior at temperatures which are below its degradation temperature, the radial block copolymer comprising from about 4 to 8 arms, wherein each of the arms has a molecular weight of from about 7,000 to about 15,000, and wherein the arms comprise blocks of styrene and hydrogenated 1,3-butadiene; and wherein the styrene is present in an amount of from about 20 to about 30 wt % of the radial block copolymer.